In NMR apparatus the probe coil is the means for transferring excitation energy to the sample and for picking up the signal emitted by de-exciting nuclei. The coil is carefully designed to be tuned for both functions and to disturb the homogeneity of the sample region as little as possible. The tuned coil should exhibit a high Q and must be carefully impedance matched to the preamplifier of the receiver and the RF source. The preamplifier is physically located at some distance from the coil and necessitates a cable in between, the latter having the characteristic impedance to match to devices placed at both its input and its output.
It is desirable to have the facility to tune rf transmitter/receiver coil(s) of an rf spectroscopic apparatus from a remote location, e.g., to avoid mechanical adjustments proximate the coil which may affect the rf properties of the coil in unexpected and uncontrolled fashion. In many applications, the accessibility of the rf coil is inconvenient for such adjustment in any event. Probe tuning is a major factor in the efficient operation of NMR apparatus. Such efficiency relates to energy transfer to and from the probe and depends largely on the degree to which the probe is electrically matched to the receiver preamplifier and/or the rf source.
In the prior art it is known to employ length of coaxial cable of length d=.lambda./2 connecting the coil to a remote tuning arrangement. This approach was discussed by Cross, Hester and Waugh, Rev. Sci. Inst., v. 47, pp. 1486 or 1488 (1976), as half of a double tuned single coil. Gordon and Tims, J. Mag. Res., v. 46, pp. 322-324 (1982), have discussed a similar arrangement wherein a length of cable and remote tuning capacitors are used to remotely tune a coil, which has first been tuned locally to one frequency, to a different second frequency. A pair of remote tuning capacitors are displaced remotely by an intervening length of cable d, chosen such that the remote tuning capacitors, arranged as a half-T network are of standard values given the cable impedance, the cable length d, the local capacitances and inductance of the coil.
In the present invention, the gross power dissipation is distributed between the external tuning means, the transmission line and the coil so as to minimize such dissipation in the external (remote) tuning means and cable and to maximize same in the rf coil. In order to accomplish this it is recognized that the efficiency with which rf power is transferred over the cable is managed advantageously by choosing local fixed capacitances at the rf coil which effect a small reduction of the reflection coefficient which obtains between the transmission line and the rf coil.